HR: 1340h
AN: A43A-0877    [Abstracts]
TI: Observations and Modelling of Aircraft HOx Measurements Over West Africa
AU: * Commane, R
EM: chmrco@leeds.ac.uk
AF: School of Chemistry, University of Leeds, Leeds, LS2 9JT, United Kingdom
AU: Floquet, C F
EM: cfaf@noc.soton.ac.uk
AF: School of Chemistry, University of Leeds, Leeds, LS2 9JT, United Kingdom
AU: Ingham, T
EM: T.Ingham@leeds.ac.uk
AF: School of Chemistry, University of Leeds, Leeds, LS2 9JT, United Kingdom
AU: Heard, D E
EM: D.E.Heard@leeds.ac.uk
AF: School of Chemistry, University of Leeds, Leeds, LS2 9JT, United Kingdom
AU: Evans, M J
EM: mat@env.leeds.ac.uk
AF: School of Earth & Environment, University of Leeds, Leeds, LS2 9JT, United Kingdom
AU: AMMA Science Team, U
EM: D.E.Heard@leeds.ac.uk
AB: As the primary oxidant in the troposphere, the hydroxyl (OH) radical controls the processing of anthropogenic and biogenic emissions. Methane is the most abundant trace gas in the atmosphere, with ~80% of global methane being removed in tropical regions by reaction with OH. However, to date, measurements of OH in the tropical boundary layer and free troposphere have been sparse. Due to the fast intercoversion of OH and HO₂, the simultaneous measurement of both species is desirable. In support of the African Monsoon Multidisciplinary Analyses (AMMA) mission occurring in West Africa, the UK FAAM (Facility for Airborne Atmospheric Measurements) BAe-146 aircraft was deployed to Niger during the summer of 2006. Alongside other observations, the first measurements of OH and HO₂ in this understudied, but very important, region were made using an airborne Fluorescence Assay by Gas Expansion (FAGE) instrument. At an altitude of 1100 m and for a signal-to-noise ratio of 1, the average limit of detection for OH and HO₂ were 7.2 × 10⁵ molecule cm^-3 (30 s integration time) and 3.1 × 10⁶ molecule cm^-3 (1 s integration time) respectively. OH was measured on 7 flights and HO₂ on 13 flights, with a large range of air mass types encountered. Observations of OH and HO₂ show great variability. A series of day and night flights on one day show HO₂ concentrations varying from a maximum of 8 × 10⁸ molecule cm^{- 3} at solar noon to a minimum of 1 × 10⁷ molecule cm^-3 at night. The distribution of OH over West Africa is discussed in terms of its spatial and temporal variation, with emphasis given to the differences in concentrations observed in monsoonal, Saharan and anthropogenic air. Box model calculations constrained by observations are used to investigate the mechanisms controlling the HO_x concentrations. These calculations suggest that much of the variability observed in OH and HO₂ can be attributed to the large variations in J(O¹D), O₃ and H₂O observed. However, in biogenically perturbed forested regions north of the Gulf of Guinea, isoprene plays a significant role in controlling the HO_x concentrations observed.
DE: 0317 Chemical kinetic and photochemical properties
DE: 0322 Constituent sources and sinks
DE: 0365 Troposphere: composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting